Abstract

Robotic devices, such as rovers and autonomous
spacecraft, have been successfully controlled by plan
execution systems that use plans with temporal flexibility to
dynamically adapt to temporal disturbances. To date these
execution systems apply to discrete systems that abstract
away the detailed dynamic constraints of the controlled
device. To control dynamic, under-actuated devices, such
as agile bipedal walking machines, we extend this execution
paradigm to incorporate detailed dynamic constraints.
Building upon prior work on dispatchable plan execution,
we introduce a novel approach to flexible plan execution of
hybrid under-actuated systems that achieves robustness by
exploiting spatial as well as temporal plan flexibility. To
accomplish this, we first transform the high-dimensional
system into a set of low dimensional, weakly coupled
systems. Second, to coordinate these systems such that they
achieve the plan in real-time, we compile a plan into a
concurrent timed flow tube description. This description
represents all feasible control trajectories and their temporal
coordination constraints, such that each trajectory satisfies
all plan and dynamic constraints. Finally, the problem of
runtime plan dispatching is reduced to maintaining state
trajectories in their associated flow tubes, while satisfying
the coordination constraints. This is accomplished through
an efficient local search algorithm that adjusts a small
number of control parameters in real-time. The first step
has been published previously; this paper focuses on the last
two steps. The approach is validated on the execution of a
set of bipedal walking plans, using a high fidelity simulation
of a biped.